(1) Field of the Invention
This invention relates to a resin material for a plastic lens and also to a lens composed thereof.
(2) Description of the Prior Art
A plastic lens is advantageous over a glass lens in that the plastic lens is easier to make, and when it is used as a spectacle lens, a high safety is assured because it is light in weight and has a good impact resistance.
An acrylic resin, a polycarbonate resin, an allyl diglycolcarbonate resin, a polystyrene resin and the like have heretofore been used as a resin material for a plastic lens. Among these resins, diethylene glycol bisallylcarbonate, which is a thermosetting resin, is widely used as a plastic lens for a sight-accomodating spectacle. This resin has various advantageous characteristics. For example, the impact resistance is excellent, the degree of the spectacle is not greatly varied depending upon the change of the temperature, and the resin is excellent in the cutting property and processability.
However, the refractive index of diethylene glycol bisallylcarbonate is 1.50 and a plastic lens made of this resin has a large thickness on the peripheral edge portion thereof than that of a glass lens. This defect is prominent as the degree of the lens is increased.
Various resins for plastic lenses have recently been proposed as resin materials free of the above-mentioned defect. For example, British Pat. No. 2,034,721 teaches that a copolymer of a dimethacrylate having a bisphenol group with chlorostyrene has a refractive index of 1.591. Furthermore, Japanese Unexamined Patent Publication (KOKAI) No. 166214/81 teaches that a copolymer of a dimethacrylate having a tetrabromobisphenol group with chlorostyrene has a refractive index of 1.603. Although these resins have a considerably high refractive index, they possess poor toughness and impact resistance.
As means for imparting a good toughness and an appropriately high refractive index, there is known a method comprising mixing or copolymerizing the resin with polyurethane. For example, U.S. Pat. No. 4,360,653 teaches that a copolymer of polyol(allylcarbonate) with polyurethane having a terminal acrylate functionality is excellent in strength, impact resistance and optical clarity, and Laid-Open EPC Application No. 59,561A teaches that a reaction product of hydroxymethacrylate and xylene diisocyanate has a refractive index of at least 1.55. However, these resins do not have a high refractive index sufficient to provide a sight-accommodating lens.